Stray magnetic shielding for a non-volatile MRAM

ABSTRACT

A non-volatile magneto-resistive memory positioned on a semiconductor substrate is shielded from stray magnetic fields by a passivation layer partially or completely surrounding the non-volatile magneto-resistive memory. The passivation layer includes non-conductive ferrite materials, such as Mn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO, FeO, or NiFeO, for shielding the non-volatile magneto-resistive memory from stray magnetic fields. The non-conductive ferrite materials may also be in the form of a layer which focuses internally generated magnetic fields on the non-volatile magneto-resistive memory to reduce power requirements.

FIELD OF THE INVENTION

The present invention pertains to non-volatile magneto-resistivememories and more specifically to passivation of non-volatilemagneto-resistive memories.

BACKGROUND OF THE INVENTION

Very high density non-volatile memories utilizing GiantMagneto-Resistive (GMR) materials as the memory element integrated withCMOS devices or circuits have been proposed. These memories operate bystoring information as the orientation of the magnetization vectors inthe GMR memory elements. The magnetization vectors are oriented by meansof applied magnetic (H) fields. The magnetic fields used to read andwrite the orientation of the magnetization vectors are generated by theintegrated CMOS circuitry. Stray magnetic fields, generated external tothe memory, having sufficient magnitude could cause errors in memoryretention.

Stray or externally generated magnetic fields can come from an almostinfinite number of sources. Stray magnetic fields having sufficientmagnitude may cause the magnetization vectors stored in a magneticmemory to change uncontrollably. High density non-volatilemagneto-resistive memories are especially sensitive to stray fieldsbecause the cells are becoming very small and, hence, require relativelylow fields for reading and writing (switching or sensing the magneticvectors). Also, stray fields from neighboring cells become larger as thedistance between neighboring cells decreases at high densities.

One method of avoiding sensitivity to stray magnetic fields is to designthe memory elements or cells such that they require higher switchingfields than the stray fields they would encounter in commercial ormilitary use. This method requires an exhaustive characterization of thestray fields encountered in these applications. This method also setshigher limits of internal power requirements to operate the memory,since higher internal fields requires more power to generate the higherinternal fields, making them high power devices and, therefore, lessdesirable. The market for high density memories is extremelycompetitive. Cost differentials between suppliers of milli-cents per bitcan mean success or failure in the market place. The addition ofmanufacturing steps or increased packaging complexity adds cost to theproduct which could determine the competitiveness of the product in themarket place.

Accordingly it is highly desirable to provide a non-volatilemagneto-resistive memory which is shielded from stray magnetic fieldswithout adding substantial cost to the memory.

It is a purpose of the present invention to provide a new and improvednon-volatile magneto-resistive memory with stray magnetic shielding.

It is another purpose of the present invention to provide a new andimproved non-volatile magneto-resistive memory with stray magneticshielding which does not add substantially to the cost of the memory.

It is still another purpose of the present invention to provide a newand improved non-volatile magneto-resistive memory with stray magneticshielding which is incorporated into the standard passivation technique.

It is a further purpose of the present invention to provide a new andimproved non-volatile magneto-resistive memory with stray magneticshielding which also focuses the internally generated magnetic fields.

It is a still further purpose of the present invention to provide a newand improved non-volatile magneto-resistive memory with stray magneticshielding which also focuses the internally generated magnetic fields soas to reduce the amount of power required to operate the memory.

It is a still further purpose of the present invention to provide a newand improved non-volatile magneto-resistive memory with focusing of theinternally generated magnetic field.

SUMMARY OF THE INVENTION

The above problems and others are at least partially solved and theabove purposes and others are realized in a non-volatilemagneto-resistive memory with stray magnetic shielding including anon-volatile magneto-resistive memory positioned on a substrate and apassivation layer at least partially surrounding the non-volatilemagneto-resistive memory. The passivation layer includes ferritematerials for shielding the non-volatile magneto-resistive memory fromstray magnetic fields, examples of the ferrite material including:Mn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO, FeO, and NiFeO.

Various specific applications of the ferrite shielding includeintermixing powdered ferrite in a layer of passivation material toprovide shielding against stray magnetic fields and forming a layer offerrite material over the non-volatile magneto-resistive memory to focusinternally generated magnetic fields as well as shielding thenon-volatile magneto-resistive memory against stray magnetic fields.Focusing the internally generated magnetic fields reduces the amount ofinternally generated magnetic field required for switching and sensing,which in turn reduces the amount of operating power used by thenon-volatile magneto-resistive memory.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings:

FIG. 1 is a simplified and enlarged cross-sectional view of a portion ofa non-volatile magneto-resistive memory in accordance with the presentinvention;

FIG. 2 is a graphical representation of magnetic fields required forswitching states in the non-volatile magneto-resistive memory of FIG. 1;and

FIG. 3 is a simplified cross-sectional view of a high densitynon-volatile magneto-resistive memory in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 is an enlarged cross-sectional viewof an example of a giant magnetoresistive GMR cell 10 having multiplelayers that are ferromagnetically coupled. Non-volatilemagneto-resistive memory cell 10 is used in this explanation only forexample and it will be understood by those skilled in the art that anyof a variety of non-volatile magneto-resistive memory cells can be usedin conjunction with the present structure. Cell 10 has a plurality ofmagnetic layers including a first magnetic layer 11 and a secondmagnetic layer 13. Layers 11 and 13 are separated by a first conductivespacer layer 12. Magnetic layers 11 and 13 each can be single layers ofmagnetic materials or, alternately, can be a composite magnetic layer.Additionally, layer 11 has a first thickness or thickness 23 and layer13 has a second thickness or thickness 24 that is greater than thickness23.

In this example, layers 11 and 13 are rectangular and are formed withthe easy axis of magnetization along a length 27 and not along a width26. In other types of devices, the easy axis can be along width 26.Layers 11 and 13 each have magnetization vectors 21 and 22 that aresubstantially along length 27, that is, substantially parallel to length27. Here it should be understood that only one set of vectors, 21 or 22,will be present in cell 10 but the two different states are illustratedsimultaneously in FIG. 1 for convenience. Layers 11 and 13 are coupledby a ferromagnetic coupling which allows vectors 21 and 22 to align inthe same direction in the absence of an external magnetic field. Thiscoupling is a function of the material and the thickness of layer 12.

Additionally width 26 is formed to be smaller than the width of themagnetic domain walls or transition width within layers 11 and 13.Consequently, vectors 21 and 22 cannot be parallel to width 26.Typically, widths of less than 1.0 to 1.2 microns result in such aconstraint. In this example, width 26 is less than one micron and is assmall as can be made by manufacturing technology, and length 27 isapproximately five times width 26. Also in this example, thickness 23 isapproximately three to six nanometers and thickness 24 is approximatelyfour to ten nanometers. As will be seen hereinafter, the difference inthickness 23 and 24 affects the switching points of layers 11 and 13.Vectors 21 and 22 illustrate two different states of magnetizationvectors within cell 10. One state is referred to as a logic "0" and theother state is a logic "1". For each state vectors in both layers 11 and13 point in a first direction, and for the other state vectors in bothlayers 11 and 13 point in the opposite or second direction.

To write or change the state of cell 10, a total magnetic field isapplied that is sufficient to completely switch the direction of themagnetic vectors of both layers 11 and 13 from along one direction oflength 27 to along the opposite direction of length 27, that is, toswitch from the state represented by vectors 21 to the state representedby vectors 22 or vice versa. To provide the total magnetic field, atransverse conductor or word line 16 is formed on the surface of adielectric 14 overlying memory cell 10 and a second conductor (notshown) is connected to opposite ends of cell 10 in individual rows toform a sense line. A combination of the sense line and word line 16 arealso used to read, or sense, the state stored in cell 10. It should benoted that in some cases, an additional digit line (not shown) which isperpendicular to word line 16 is required to ensure that the value ofthe total magnetic field is sufficient to cause the magnetic vectors toeither rotate or to switch. The value of the total magnetic field is asummation of the magnetic fields resulting from the sense, word, anddigit line currents.

FIG. 2 is a graph 31 illustrating the resistance or voltage output ofcell 10 (FIG. 1) verses the applied magnetic field or total magneticfield. The abscissa indicates magnetic field direction and strength,that is, the strength either supports or opposes the magnetic vectors ofcell 10. The ordinate represents the voltage output of cell 10. A curve32 indicates the magnetoresistance characteristic, via the outputvoltage, for various magnetic field intensities for one direction ofmagnetization vectors (for example vectors 21). A curve 33 indicates themagnetoresistance characteristic, via the output voltage, for the samemagnetic field intensities for the opposite direction of magnetizationvectors (for example vectors 22). To the right of zero, curves 32 and 33indicate the output voltage for magnetic fields that support the vectorsof curve 32 and oppose the vectors of curve 33, and magnetic fields tothe left of zero support the vectors of curve 33 and oppose the vectorsof curve 32. Typically, curves 32 and 33 cross the voltage axis at thesame point and have the same minimum values. For the sake ofexplanation, curve 33 is shifted vertically a slight amount to show thedifferences between the curves.

At zero applied field, the voltage output of cell 10 is approximatelythe same regardless of the magnetization vector direction. As the fieldincreases from zero to H₁, curve 33 shows the voltage output of cell 10having vectors that are opposed by the total magnetic field, and curve32 shows the voltage of cell 10 having vectors that are supported by themagnetic field. At magnetic field intensity of H₁, the vectors of thelayer 11 begin to rotate and increase the output voltage. As the totalmagnetic field intensity increases between H₁ and H₃, the magneticvectors of layer 11 continue to rotate and snap to the other directionnear a field intensity of H₃. Near H₄, the vectors of thicker layer 13snap to the opposite direction and the resistance decreases for valuesof H₄ and above. Similarly, the output voltage for an opposite directiontotal magnetic field is shown between zero and H₅ to H₈.

The resistance is normally determined by sensing a voltage output ofcell 10. The voltage output is the voltage drop across the length ofcell 10 with a constant current applied along the length of cell 10 andwhile a magnetic field is applied. One method of determining the stateof cell 10 is to apply a total magnetic field that is higher than theswitching threshold for layer 11 (i.e. H₃) but not as high as theswitching threshold for layer 13 (i.e. H₄). When the total magneticfield is in a direction that supports the magnetic vectors, that is, inthe same direction along length 27 as the magnetization vectors, themagnetic vectors do not substantially rotate so the resistance of cell10 does not substantially change. Correspondingly, the output voltagealso does not substantially change.

However, when the total magnetic field opposes the vectors, the magneticvectors rotate. As the field increases the vectors of layer 11 begin torotate toward the opposite end of layer 11 (the vectors of layer 13 mayrotate slightly). As the field increases further, the vectors of layer11 continue to rotate and the resistance increases until the vectorssnap to the opposite direction. For further increases, the resistanceremains substantially constant until the vectors of layer 13 also snap,which produces a change in stored information. Thereafter, theresistance decreases as the field increases.

Because cell 10 operates on the premise that the value of the totalmagnetic field is a summation of the magnetic fields resulting from thesense, word, and digit line currents, it can be seen that stray magneticfields combined with the total magnetic field can produce substantialerrors in either of the reading or writing operations, as well aseffecting cell 10 during normal storage. For example, during readingoperations, when a total magnetic field is higher than the switchingthreshold of layer 11 but not sufficient to switch the magnetic vectorsof layer 13, a stray magnetic field could easily supply a sufficienttotal magnetic field to surpass H₈ and actually switch the informationin the cell. Further, because many of the memory cells are very smalland packed very closely together, especially in large arrays, arelatively small amount of stray magnetic field can have a substantialeffect thereon.

To alleviate the stray magnetic field problem, a passivation layer 18 isformed at least partially surrounding memory cell 10. In a preferredembodiment, layer 18 is formed of a layer of non-conductive highpermeability material, such as ferrite material. Some ferrite materialsthat are suitable for the described purpose are at least one of:Mn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO, FeO, and NiFeO. Becauselayer 18 is non-conductive, it can be deposited directly on the surfaceof cell 10 or a thin layer 17 of dielectric material may be used betweencell 10 and layer 18, if layer 18 is sufficiently conductive to affectthe operation. Because layer 18 is formed of a high permeabilitymaterial any stray magnetic fields are shielded from cell 10. Further,any magnetic field produced by current being applied to word line 16 isfocused or directed onto cell 10 by layer 18 so that a smaller amount ofcurrent can be applied to word line 16 to achieve the same amount oftotal magnetic field required for reading and/or writing.

Thus, layer 18 performs the dual function of shielding cell 10 againststray magnetic fields and focusing internally generated magnetic fieldswithin cell 10. In the event that only the shielding function isdesired, layer 18 can be formed of a typical passivation material (anyconvenient dielectric material which provides a good barrier to externalmoisture, etc.) with a quantity of high permeability materialintermixed. Generally the high permeability material can be powdered andmixed with the passivation material in a liquid or semi-liquid form andthen applied to the cell or array or molded around the cell. Also, thehigh permeability material can be sputtered or spun-on along with thepassivation material. In yet another application technique, powders ofthe ferrite materials can be spray coated onto the passivation layer, oron the back of the substrate, or onto the package as a very low costdeposition method.

Turning now to FIG. 3, a simplified and enlarged cross-sectional view ofan array 50 of cells, similar to cell 10, is illustrated. Only a portionof array 50 is illustrated for convenience, including cells 51 and 52.Typically, a plurality of cells (e.g. 51 and 52) similar to cell 10 areformed on a common substrate 55 with a space between each individualcell 51, 52, etc. A conductor 56 is then applied to interconnect cells51, 52, etc. in individual rows (sense lines). A plurality of transverseconductors or word lines 57 are associated in overlying relationship oneeach with each column of the memory cells.

A passivation layer 60 is formed over the entire array to completelypassivate the entire array and any integrated circuits (illustrated inblock form at 61) associated with the array. Passivation layer 60, in apreferred embodiment is formed of a layer of ferrite materials which aresputtered or spun-on using any of the well known techniques. Once theentire passivation layer 60 is formed, openings 65 are formed throughpassivation layer 60 to allow connections to bonding pads and the like.In this preferred embodiment, passivation layer 60 not only passivatesand shields the array but focuses or directs internally generatedmagnetic fields (e.g. those generated by current in word lines 57) ontothe associated cells. As previously described, if only the functions ofshielding and passivation are desired, passivation layer 60 is formed ofa passivating material with powdered or small particles of a highmagnetic permeability material intermixed therein.

Thus, a new and improved non-volatile magneto-resistive memory withstray magnetic shielding has been disclosed. The stray magneticshielding does not add substantially to the cost of the memory becauseit is simple and easy to apply, generally being incorporated into aprocedure which is already in place as, for example, the standardpassivation technique. In a preferred embodiment, the stray magneticshielding for the new and improved non-volatile magneto-resistive memoryalso focuses the internally generated magnetic fields of the memory. Thefocusing of the magnetic fields, along with elimination of straymagnetic fields allows the internally generated magnetic fields to besubstantially reduced, which reduces the amount of power required tooperate the memory. Further, the focusing reduces the power consumptionof the cell, reduces the metal current density and improves theassociated metal reliability. The focusing also reduces the size ofdrive transistors to increase cell real estate efficiency.

While we have shown and described specific embodiments of the presentinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it to be understood, therefore, that thisinvention is not limited to the particular forms shown and we intend inthe appended claims to cover all modifications that do not depart fromthe spirit and scope of this invention.

What is claimed is:
 1. A non-volatile magneto-resistive memory withstray magnetic shielding comprising:a non-volatile magneto-resistivememory positioned on a substrate and defining an upper surface; and apassivation layer at least partially surrounding the non-volatilemagneto-resistive memory, the passivation layer including ferritematerials for shielding the non-volatile magneto-resistive memory fromstray magnetic fields.
 2. A non-volatile magneto-resistive memory withstray magnetic shielding as claimed in claim 1 wherein the ferritematerials include one of Mn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO,FeO, and NiFeO.
 3. A non-volatile magneto-resistive memory with straymagnetic shielding as claimed in claim 1 wherein the ferrite materialsare in the form of a powder intermixed with an encapsulation material.4. A non-volatile magneto-resistive memory with stray magnetic shieldingas claimed in claim 1 wherein the ferrite materials are in the form of alayer of the ferrite material positioned adjacent the non-volatilemagneto-resistive memory.
 5. A non-volatile magneto-resistive memorywith stray magnetic shielding comprising:a non-volatilemagneto-resistive memory positioned on a substrate and defining an uppersurface, the non-volatile magneto-resistive memory including at leastfirst and second layers of magneto-resistive material separated by alayer of non-magnetic material; a layer of high permeability,non-conductive magnetic material positioned adjacent the upper surfaceof the non-volatile magneto-resistive memory so as to focus internallygenerated magnetic fields on at least one of the first and second layersof magneto-resistive material and shield the non-volatilemagneto-resistive memory from stray magnetic fields; and a passivationlayer at least partially surrounding the layer of high permeability,non-conductive magnetic material and the non-volatile magneto-resistivememory.
 6. A non-volatile magneto-resistive memory with stray magneticshielding as claimed in claim 5 wherein the layer of high permeability,non-conductive magnetic material includes ferrite materials.
 7. Anon-volatile magneto-resistive memory with stray magnetic shielding asclaimed in claim 6 wherein the ferrite materials include one ofMn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO, FeO, and NiFeO.
 8. Anon-volatile magneto-resistive memory with stray magnetic shielding asclaimed in claim 5 wherein the passivation layer includes ferritematerials.
 9. A non-volatile magneto-resistive memory with straymagnetic shielding comprising:a non-volatile magneto-resistive memorypositioned on a semiconductor substrate, the non-volatilemagneto-resistive memory including an array of individual cells witheach cell including at least first and second layers ofmagneto-resistive material separated by a layer of non-magneticmaterial, the non-volatile magneto-resistive memory further includingintegrated circuitry addressing and controlling the individual cellswith input/output terminals; and a layer of high permeability,non-conductive magnetic material coating at least an upper surface ofthe non-volatile magneto-resistive memory so as to focus internallygenerated magnetic fields on at least one of the first and second layersof magneto-resistive material in each of the individual cells and shieldthe non-volatile magneto-resistive memory from stray magnetic fields.10. A non-volatile magneto-resistive memory with stray magneticshielding as claimed in claim 9 wherein the layer of high permeability,non-conductive magnetic material includes ferrite materials.
 11. Anon-volatile magneto-resistive memory with stray magnetic shielding asclaimed in claim 10 wherein the ferrite materials include one ofMn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO, FeO, and NiFeO.
 12. Anon-volatile magneto-resistive memory with stray magnetic shielding asclaimed in claim 9 including in addition a passivation layer at leastpartially surrounding the layer of high permeability, non-conductivemagnetic material and the non-volatile magneto-resistive memory.
 13. Anon-volatile magneto-resistive memory with stray magnetic shielding asclaimed in claim 12 wherein the passivation layer includes ferritematerials.
 14. A non-volatile magneto-resistive memory with straymagnetic shielding as claimed in claim 13 wherein the ferrite materialsinclude one of Mn--Zn-Ferrite, Ni--Zn-Ferrite, MnFeO, CuFeO, FeO, andNiFeO.
 15. A non-volatile magneto-resistive memory with stray magneticshielding as claimed in claim 13 wherein the layer of high permeability,non-conductive magnetic material includes openings therethrough foraccess to the input/output terminals.